Method for producing a multilayer coating

ABSTRACT

Process for producing a multilayer coating, in which a first coating (A) has applied to it a subsequent coating material (B) which is then cured involves selecting and/or modifying the first coating (A) and/or selecting the coating material (B) in such a way that the quotient (Q) formed from the surface energy of the second coating (B) and the surface energy of the first coating (A) is less than or equal to 1, and its use.

The present invention relates to a process for producing a multilayercoating, e.g. a multicoat paint system, in which a first coating (A) hasapplied to it a subsequent coating material (B), which is then cured,and to its use.

For a production-line vehicle paint system, in particular a high-qualityautomotive OEM finish, as is known, multicoat color and/or effect paintsystems are used which are composed of primer, electrocoat, surfacer orantistonechip primer, basecoat and clearcoat. The clearcoats must meetstringent requirements in terms of optical and esthetic properties(appearance) and of hardness, scratch resistance, chemical resistance,etch resistance, and weathering stability.

A refinish is subject to the same requirements in respect of theproperties as the OEM finish; that is, high resistance properties withrespect to the effects of weathering, to chemicals and to mechanicalloads are expected (see above). Refinishes involve alternatively anaftercoating or overcoating of an area of an automobile which has beendamaged as a result of an accident, for example, or a graduated finish,or a complete overcoating of an automobile which has already beenpainted, owing to paint work damage, color differences or other unwanteddefects in the paint already applied. The paint used for the refinishmust adhere to the topmost coat of the original finish (OEM finish) andwet it completely. The intention here is to avoid laborious mechanicalpretreatment such as sanding. For the OEM finish, the paints used forthe top and bottom coats can be matched to one another during theirpreparation, so that effective wetting and adhesion are normallyensured. Such matching is not possible in the case of refinish. First,wetting/adhesion on the topmost topcoat of the OEM finish by therefinish paint is difficult to achieve owing to the (required)properties of the topcoat. The topcoat, in fact, is highly crosslinked,apolar, unreactive and inert.

Second, the refinish paint must at the same time adhere to the lowercoats as well, if the overlying coats have flaked off. And, thirdly, therefinish paints have to be cured at relatively low temperatures, so asnot to impair parts on the vehicle that are made of plastic or rubber.Accordingly, those coating materials curable with actinic radiation orwith both actinic and thermal radiation would be preferable for suchtasks, being curable at low temperatures.

On the basis of their special properties, the use of these coatingmaterials in the automotive industry is particularly desirable. Theyexhibit particularly good gloss, high hardness, excellent weatheringstability, and good scratch resistance.

Nevertheless, the use of these coating materials as OEM paints in theautomobile industry has been hindered to date by the poor adhesion of apaint film to be applied thereto subsequently and by the inadequatewetting of the coatings produced with these coating materials.

Effective wetting of the (lower) coating by the subsequently appliedcoating material, and excellent subsequent adhesion of the cured coatingmaterial to the coating, however, are necessary in order for a furthercoating material, e.g., topcoat material, to be applied to the lowercoating or in order to carry out a refinish and to obtain a permanentbond between the coats and, accordingly, a multilayer coating of highquality and durability.

The same is true of refinish systems, and especially when refinishingmulticoat paint systems composed of primer, electrocoat, surfacer orantistonechip primer, basecoat, and clearcoat. Thus, in the case forexample of only slight damage to the clearcoat, refinishing requires thelatter to be overcoated with itself, with attendant problems of wettingand of subsequent adhesion (see above) as a result of the differentproperties of the cured clearcoat and of the liquid clearcoat materialstill to be applied. These problems are exacerbated when not only theclearcoat but also other, underlying coats have flaked off and mustlikewise be refinished or reconstructed in order to maintain the overallappearance.

EP 0349749 A1 discloses the use of a plasma pretreatment of paintedcomponents in order to enhance the adhesion properties of a second paintcoat to be applied subsequently. As to what the ratio of the surfacetensions should be, nothing is said. Nor is there any disclosure of itsapplication to coatings cured with actinic radiation or both thermallyand with actinic radiation.

It is an object of the present invention, therefore, to provide a novelprocess for producing multilayer coatings which no longer has thedisadvantages of the prior art but which instead can be employedsubstantially independently of the prevailing conditions, particularlyas regards temperature and atmospheric humidity, and even under extremeconditions. In such a process, any coat to be applied subsequentlyshould adhere well to the previous coat, and should also wet itcompletely.

In particular, the new process ought to allow the coating to berefinished, and the refinished area thus obtained ought not to sufferany damage and to give a durable refinish of high quality at both highand low temperatures, high and low atmospheric humidity, and also underconditions fluctuating rapidly between these extremes, such as aredominant in tropical and desert climates, under high radiative intensityand under intensive mechanical and chemical loads, irrespective of thelayer of the multilayer coating to which the coating material used forrefinishing is applied.

The novel process ought in particular to be reliably applicable over aslarge a selection of coatings and coating materials as possible,particular attention being placed on the coating materials curable orcoatings cured by means of actinic radiation.

The invention accordingly provides a process for producing a multilayercoating, in which a first coating (A) has applied to it a subsequentcoating material (B) which is then cured, which involves selectingand/or modifying the first coating (A) and/or selecting the coatingmaterial (B) in such a way that the quotient (Q) formed from the surfaceenergy of the second coating (B) and the surface energy of the firstcoating (A) is less than or equal to 1.

The quotient Q is calculated by dividing the surface energy of thesecond coating (B) by the surface energy of the coating (A).

The process of the invention allows effective wetting of the lowercoating (A) by the subsequently applied coating material (B) and alsoexcellent subsequent adhesion of the coating (B) to the coating (A).

Furthermore, as a result of the process of the invention, it becomespossible to produce a multilayer coating substantially independently ofthe prevailing conditions, particularly as regards temperature andatmospheric humidity, and even under extreme conditions. Any coat to beapplied subsequently adheres well to the previous coat and wets itcompletely.

In addition, the refinishability of the coating is enhanced as a resultof the novel process. The refinished area obtained in this way isdurable under high and low temperatures, high and low atmospherichumidity, and also under conditions fluctuating rapidly between theseextremes, such as occur in tropical or desert climates, and suffers nodamage under high radiative intensity and under intense mechanical andchemical load, but instead produces a durable refinish of high quality,irrespective of the coat of the multilayer coating to which the coatingmaterial is applied.

Furthermore, the process of the invention makes overcoating orrefinishing successful, since wettability and subsequent adhesion areguaranteed. Indeed, through the teaching according to the invention, thecoating practitioner is instructed that he or she can ensure the successof his or her coating in terms of wetting and adhesion by setting thequotient Q at a value of less than or equal to 1, preferably less thanor equal to 0.95, and in particular 0.9.

The quotient Q can be set by selecting and/or modifying the coating (A)and/or the coating material (B), such as is normally done in the case ofan original, OEM finish composed of basecoat and clearcoat.

Should this not be possible or desirable, because, for example, adifferent appearance is produced or over-coating of the coating materialwith itself is required, the quotient Q can also be set by modifying thecoating (A), in particular the surface of the coating (A). For thispurpose it is possible to employ one or a combination of the followingsurface treatment methods: low-pressure plasma technology,atmospheric-pressure plasma technology, flaming, fluorination,silicatization.

Further, the coating (A) may be treated with liquid primers by means,for example, of dipping, spraying or brushing. It is also possible touse dielectric barrier discharge (corona) for the surface treatment.

The methods referred to are familiar to the skilled worker and may befound in the following references (Römpp Lexikon Lacke und Druckfarben,Georg Thieme Verlag Stuttgart, 1998, page 416 “Surface tension”), plasmatreatment (Römpp Lexikon Lacke und Druckfarben, Georg Thieme VerlagStuttgart, 1998, page 455 “Plasma treatment”, PLASMA-TREAT®, brochure,AGRODYN Hochspannungstechnik GmbH), flaming (Römpp Lexikon Lacke undDruckfarben, Georg Thieme Verlag Stuttgart, 1998, page 59 “Flaming”;Automatic flamer model S 4S 300/2000 from Friedrich SchäferMaschinenbaugesellschaft mbH, Sprendlingen), Fluorination (Römpp LexikonLacke und Druckfarben, Georg Thieme Verlag Stuttgart, 1998, page 0.244“Fluorination”), silicatization, primer coating (Römpp Lexikon Lacke undDruckfarben, Georg Thieme Verlag Stuttgart, 1998, page 472 “Primer”),dielectric barrier discharge (Römpp Lexikon Lacke und Druckfarben, GeorgThieme Verlag Stuttgart, 1998, page 117 “Corona”).

In one particularly preferred variant of the process the surface energyof the first coating (A) is modified and/or selected, in order to setthe quotient Q, such that it is >30, preferably >40, and inparticular >50 mJ/m². In that case, particularly good wetting andsubsequent adhesion are likewise obtained.

Surface tension is a name for the interfacial tension of solids andliquids with respect to the vapor phase or air. It is defined as forceper unit length, has the dimension mN/m, and in terms of dimension andvalue is equal to the surface energy required either actually to formthe surface or to increase it under reversible conditions andisothermally. Under certain conditions, the surface tension correspondsto the free energy of the surface per unit area (surface energy inmJ/m²). The surface energy of solids can be measured, inter alia, bydetermining the contact angle of liquid drops of known surface tensionand polarity and by evaluating the measurements by the method of Kaelbleor Zismann (Römpp Lexikon Lacke und Druckfarben, Georg Thieme VerlagStuttgart, 1998, page 416, “Surface tension”; CD Römpp ChemieLexikon—Version 1.0, Stuttgart/New York; Georg Thieme Verlag 1995“Wetting”). Other methods are known from “Lackadditive” [Additives forCoatings], Johan Bieleman, Weinheim, WILEY-VCH 1998, page 133 ff.

The process can be carried out with the normal coatings and coatingmaterials that are known to the skilled worker. By way of example,mention may be made of alkyd resin coating materials, dispersion coatingmaterials, epoxy resin coating materials, polyurethane coatingmaterials, and acrylic resin coating materials. The coating materialsmay be used in liquid, paste or powder form. Nor are there anyparticular requirements imposed on the way in which these coatingmaterials are applied. They may be applied, for example, by spraying,knife coating, brushing, flow coating, dipping or rolling.

In particular, the process can be carried out with coatings (A) curedwith actinic radiation despite the fact that these are particularlyhighly crosslinked, apolar, unreactive and inert, and are thereforedifficult to coat without the process of the invention.

Actinic radiation suitably includes electromagnetic radiation andcorpuscular radiation. The electromagnetic radiation encompasses nearinfrared (NIR), visible light, UV radiation, X-rays, and gammaradiation, especially UV radiation. The corpuscular radiationencompasses electron beams, alpha radiation, proton beams, and neutronbeams, especially electron beams.

Coatings (A) cured with actinic radiation are produced from coatingmaterials (A) curable with actinic radiation, which, as is known,comprise radiation-curable, low molecular mass, oligomeric and/orpolymeric compounds, preferably radiation-curable binders, based inparticular on ethylenically unsaturated prepolymers and/or ethylenicallyunsaturated oligomers, further comprising, if desired, one or morereactive diluents, and also, if desired, one or more photoinitiators.Examples of suitable radiation-curable binders are(meth)acryloyl-functional (meth)acrylic copolymers, polyether acrylates,polyester acrylates, unsaturated polyesters, epoxy acrylates, urethaneacrylates, amino acrylates, melamine acrylates, silicone acrylates, andthe corresponding methacrylates. It is preferred to use binders whichare free from aromatic structural units.

Suitable UV-curable coating materials (A) are disclosed in, for example,patents EP-A-0 540 884, EP-A-0 568 967 or U.S. Pat. No. 4,675,234.Further examples of suitable coating materials curable with actinicradiation include those known from, for example, German patent DE 197 09467 C1, page 4, line 30, to page 6, line 30, or German patentapplication DE 199 47 523 A1.

Where the coating material (A) used is curable not only by actinicradiation but also thermally, i.e. is a dual-cure coating material, itpreferably further comprises conventional thermosetting binders andcrosslinking agents and/or thermosetting reactive diluents, as isdescribed in, for example, German patent applications DE 198 187 735 A1and DE 199 20 799 A1 or in European patent application EP 0 928 800 A1.

In the context of the present invention “thermal curing” means theheat-initiated curing of a film of a coating material, for whichnormally a separate crosslinking agent is employed. By those in the artthis is commonly referred to as external crosslinking. Where thecrosslinking agents are already incorporated in the binders, the termself-crosslinking is used. In accordance with the invention externalcrosslinking is of advantage and is therefore employed with preference.

The coating materials used to produce coatings (A) can also be used ascoating materials (B). Otherwise it is also possible to use coatingmaterials curable thermally and/or with actinic radiation. It ispreferred to use the coating materials (A).

EXAMPLES Example 1 Production of Coatings (A) and Determination ofSurface Tension

A conventional UV-curable varnish (AI) consisting of: 35.31% by weight Ebecryl ® 1290 (hexafunctional aliphatic urethane acrylate) 35.31% byweight  Sartomer ® 494 (ethoxylated pentaerythritol tetraacrylate) 8.65%by weight hydroxypropyl acrylate 0.98% by weight Actilane ® 800(radiation-curing silicone acrylate from Akcros Chemie) 0.14% by weightDow Corning ® PA 57 (silicone additive from Dow Corning) 0.42% by weightIrgacure ® 819 (bisacylaphosphine photoinitiator) 2.65% by weightGenocure ® MBF (photoinitiator) 1.12% by weight Tinuvin ® 123(aminoether HALS from Ciba Specialty Chemicals) 1.40% by weightTinuvin ® 400 (UV absorber from Ciba Specialty Chemicals) 5.09% byweight ethyl acetate 5.72% by weight butyl acetate 98/100% 3.21% byweight isopropanolwas cured first at RT for 20 min then for 1 min using a hand lamp(handlamp UV-H 250 from Küthnast Strahlungstechnik, Wächtersbach) at adistance of 30 cm, and subsequently in an IST inert unit at 14 m/s withpower of 4×500 mJ/cm². This gave a coating (AI).

A conventional, UV- and heat-curable varnish (AII) consisting of thefollowing constituents: Stock varnish: Methacrylate copolymer^(a)) 9Dipentaerythritol pentaacrylate 20 UV absorber (substitutedhydroxyphenyltriazine) 1.0 HALS (N-methyl-2,2,6,6-tetramethylpiperidinyl1.0 ester) Wetting agent (Byk ® 306 from Byk Chemie) 0.4 Butyl acetate27.4 Solventnaphtha ® 12.8 Irgacure ® 184 (commercial photoinitiatorfrom 1.0 Ciba Specialty Chemicals) Lucirin ® TPO (commercialphotoinitiator from 0.5 BASF AG) Total: 100 Crosslinking component:Crosslinking agent 1: Isocyanatoacrylate Roskydal ® UA VPLS 2337 27.84from Bayer AG (based on Trimeric hexamethylene diisocyanate; isocyanategroup content 12% by weight) Crosslinking agent 2: IsocyanatoacrylateRoskydal ® UA VP FWO 3003-77 6.96 from Bayer AG (based on trimer ofisophorone diisocyanate (70.5% in butyl acetate; viscosity: 1500 mPas;isocyanate group content 6.7% by weight)) Diluent 3.48 Total: 38.28

-   a) The methacrylate copolymer was prepared by the following    procedure:    -   A suitable reactor equipped with a stirrer, two dropping funnels        for the monomer mixture and initiator solution, a nitrogen inlet        tube, thermometer, heating and reflux condenser was charged with        650 parts by weight of an aromatic hydrocarbon fraction having a        boiling range of 158 to 172° C. The solvent was heated to        140° C. Then a monomer mixture of 652 parts by weight of        ethylhexyl acrylate, 383 parts by weight of 2-hydroxyethyl        methacrylate, 143 parts by weight of styrene, 212 parts by        weight of 4-hydroxybutyl acrylate and 21 parts by weight of        acrylic acid was metered into the initial charge at a uniform        rate over the course of four hours and an initiator solution of        113 parts by weight of the aromatic solvent and 113 parts by        weight of tert-butylperethylhexanoate was metered into the        initial charge at a uniform rate over the course of 4.5 hours.        The metering of the monomer mixture and of the initiator        solution was commenced simultaneously. After the end of the        initiator feed, the resultant reaction mixture was heated with        stirring at 140° C. for two hours more and then cooled. The        resulting solution of the methacrylate copolymer (A) was diluted        with a mixture of 1-methoxyprop-2-yl acetate, butylglycol        acetate and butyl acetate        was cured first at RT for 5 min, then for 10 min at 80° C., and        subsequently for 20 min at 140° C. in an IST inert unit at 14        m/s with a power of 1500 mJ/cm². This gave a coating (AII).

The two coatings (AI) and (AII) were subjected to a contact anglemeasurement as per the Krüss GmbH Hamburg, Handbook “Drop ShapeAnalysis” in accordance with the method of Owens, Wendt, Rabel, andKaeble at 23° C. and 50% relative atmospheric humidity, with thefollowing measurement liquids: double-distilled water, 1,5-pentanediol,diiodomethane, ethylene glycol and glycerol, in each case with andwithout flaming, measurement taking place in each case immediately,after one day or after four days. The surface energy was calculated fromthe contact angles measured. Sample Coating 1 5 min RT, no flaming 2AII, flaming, measured immediately 3 AII, flaming, measured after 1 day4 AII, flaming, measured after 4 days 5 AI no flaming 6 AI, flaming,measured immediately 7 AI, flaming, measured after 1 day 8 AI, flaming,measured after 4 days

Flaming was carried out with an automatic flamer model S 4-S 300/2000from Friedrich Schäfer Maschinenbaugesellschaft mbH, Sprendlingen, usinga propane gas flame of 10 cm in width at a distance of 10 cm from thesubstrate, in one pass at an advancement rate of 150 m/s.

Table 2 lists the resultantly calculated surface energies of thecorrespondingly treated coatings (AI) and (AII). TABLE 1 Contact anglesContact angle [°] Ethylene 1,5- Sample H₂O glycol Pentanediol CH₂I₂Glycerol 1 93 ± 0.4 75 ± 0.4 66 ± 0.2 61 ± 0.2 89 ± 1.4 2 42 ± 0.9 16 ±4.4 19 ± 4.2 39 ± 1.3 43 ± 0.8 3 48 ± 1.3 22 ± 1.7 21 ± 1.6 40 ± 1.9 57± 1.4 4 57 ± 1.0 32 ± 1.0 32 ± 1.0 43 ± 0.9 61 ± 1.1 5 96 ± 0.8 84 ± 0.477 ± 0.2 70 ± 0.3 96 ± 0.5 6 44 ± 4.6 29 ± 4.3 35 ± 3.7 50 ± 1.3 48 ±3.3 7 60 ± 9.5 41 ± 2.3 36 ± 1.0 52 ± 1.4 55 ± 6.3 8 66 ± 3.1 49 ± 1.249 ± 3.7 55 ± 0.8 59 ± 6.4

TABLE 2 Surface energies Surface energies [mJ/m²] Sample Dispersecomponent Polar component Total 1 23.4 1.7 25.1 2 29.4 22.4 51.8 3 28.918.5 47.4 4 29.0 14.2 43.2 5 17.4 2.0 19.4 6 24.0 24.0 48.0 7 26.5 14.741.2 8 25.0 12.0 37.0

The results show an increase in the surface energy of the coatings (AI)and (AII), i.e., of the coating (A), as a result of the flaming,irrespective of whether the coating material was curable solely withactinic radiation or both thermally and with UV radiation. The increaseis achieved in particular by raising the polar component of the surfaceenergy.

Example 2 Overcoatability of the Coating (AI), Production of aMultilayer Coating

The overcoatability of the coating (AI) with itself was examined bymeans of a cross-cut test to DIN ISO 2409:1994-10. For this purpose thecoating (AI) was overcoated with the varnish (AI), i.e., with itself,both after flaming and without flaming.

The abovementioned components constituting the UV-curable varnish (AI)were mixed with intensive stirring using a dissolver or a stirrer inorder to prepare the corresponding varnish (AI). An applied film of thisvarnish (AI) was produced with a film thickness of 40±10 μm on asuitable test panel. The film was cured first at RT for 20 min, then for1 min with a hand lamp UV-H 250 from Kühnast Strahlungstechnik,Wächtersbach, at a distance of 30 cm, and subsequently in an IST inertunit at 14 m/s with a power of 4×500 mJ/cm².

The cured varnish I (coating (AI)) (which becomes coating B) possessed asurface energy of 19.4 mJ/m².

Flaming was carried out as indicated above. The surface energy of thecoating (AI) (which becomes coating A) was now 48.0 mJ/cm².

The quotient Q=B/A was therefore 0.41.

Subsequently the above-produced coating (AI) was covered in each casewith a further film of varnish (AI) (coating material (B)) with a filmthickness of 40±10 μm. The upper film was cured, as above, initially atRT for 20 min, then for 1 min with a hand lamp UV-H 250 from KühnastStrahlungstechnik, Wächtersbach, at a distance of 30 cm, andsubsequently in an IST inert unit at 14 m/s with a power of 4×500mJ/cm².

In the case of the test panels without flaming that were investigated,cross-cut indexes of GT 4 or GT 5 were obtained. In contrast, the testpanels treated by flaming gave cross-cut indexes of GT 0 or GT 1.

Example 3 Overcoatability of the Coating (AII), Production of aMultilayer Coating

The overcoatability of the coating (AII) with itself was examined inanalogy to Example 2 above by means of a cross-cut test to DIN ISO2409:1994-10. For this purpose the coating (AII) was overcoated with thevarnish (AII), i.e., with itself, both after flaming and withoutflaming.

The cured varnish II (coating (AII)) (which becomes coating B) possesseda surface tension of 25.1 mJ/m².

Flaming was carried out as indicated above. The surface energy of thecoating (AII) (which becomes coating A) was now 51.8 mJ/cm².

The quotient Q=B/A was therefore 0.5.

Subsequently the above-produced coating (AII) was covered in each casewith a further film of varnish (AII) (coating material (B)) with a filmthickness of 40±10 μm. The upper film was cured, as above, initially atRT for 5 min, then for 10 min at 80° C. and subsequently for 20 min at140° C. in an IST inert unit at 14 m/s with a power of 1500 mJ/cm².

In the case of the test panels without flaming that were investigated,cross-cut indexes of GT 4 or GT 5 were obtained. In contrast, the testpanels treated by flaming gave cross-cut indexes of GT 0 or GT 1.

Accordingly it has been shown that it is possible, surprisingly by meansof the process of the invention to predict the success of producing themultilayer coating by setting the quotient Q.

1. A process for producing a multilayer coating, in which a firstcoating (A) has applied to it a subsequent coating material (B) which isthen cured, which comprises at least one of selecting or modifying thefirst coating (A) or selecting the coating material (B) in such a waythat the quotient (Q) formed from the surface energy of the secondcoating (B) and the surface energy of the first coating (A) is less thanor equal to 1
 2. The process as claimed in claim 1, wherein the quotient(Q) is set by modifying the coating (A).
 3. The process as claimed inclaim 2, wherein the quotient (Q) is set by modifying the surface of thecoating (A).
 4. The process as claimed in claim 3, wherein the quotient(Q) is set by raising the surface energy of the first coating (A) bymeans of at least one of the following methods: low-pressure plasmatechnology, atmospheric-pressure plasma technology, flaming,fluorinating or silicatization.
 5. The process as claimed in claim 1,wherein the quotient (Q) is set such that it is less than or equal to0.95.
 6. The process as claimed in claim 1, wherein the quotient (Q) isset such that it is less than or equal to 0.90.
 7. The process asclaimed in claim 1, wherein to set the quotient (Q) the surface energyof the first coating (A) is selected or changed such that it is >30mJ/m².
 8. The process as claimed in claim 1 wherein to set the quotient(Q) the surface energy of the first coating (A) is selected or changedsuch that it is >40 mJ/m².
 9. The process as claimed in claim 1, whereinto set the quotient (Q) the surface energy of the first coating (A) isselected or changed such that it is >50 mJ/m².
 10. The process asclaimed in claim 1, wherein at least one of the coating (A) or thecoating material (B) is cured by means of actinic radiation
 11. Theprocess as claimed in claim 1 for at least one of producing orrefinishing an automotive (OEM) finish.